Navigation

The astropy.time package provides functionality for manipulating times and
dates. Specific emphasis is placed on supporting time scales (e.g. UTC, TAI, UT1) and
time representations (e.g. JD, MJD, ISO 8601) that are used in astronomy.
It uses Cython to wrap the C language SOFA time and calendar
routines. All time scale conversions are done by Cython vectorized versions
of the SOFA routines and are fast and memory efficient.

The basic way to use astropy.time is to create a Time
object by supplying one or more input time values as well as the time format and
time scale of those values. The input time(s) can either be a single scalar like
"2010-01-0100:00:00" or a list or a numpy array of values as shown below.
In general any output values have the same shape (scalar or array) as the input.

In astropy.time a “time” is a single instant of time which is
independent of the way the time is represented (the “format”) and the time
“scale” which specifies the offset and scaling relation of the unit of time.
There is no distinction made between a “date” and a “time” since both concepts
(as loosely defined in common usage) are just different representations of a
moment in time.

Once a Time object is created it cannot be altered internally. In code lingo
it is immutable. In particular the common operation of “converting” to a
different time scale is always performed by returning a copy of the original
Time object which has been converted to the new time scale.

The time format specifies how an instant of time is represented. The currently
available formats are can be found in the Time.FORMATS dict and
are listed in the table below. Each of these formats is implemented as a class
that derives from the base TimeFormat class.
This class structure can be easily adapted and extended by users for
specialized time formats not supplied in astropy.time.

The time format classes TimeISO,
TimeISO, and
TimeYearDayTime support the concept of
subformats. This allows for variations on the basic theme of a format in both
the input string parsing and the output.

The supported subformats are date_hms, date_hm, and date. The
table below illustrates these subformats for iso and yday formats:

A Time object can hold either a single time value or an array of time values.
The distinction is made entirely by the form of the input time(s). If a Time
object holds a single value then any format outputs will be a single scalar
value, and likewise for arrays.

The Time class initializer will not accept ambiguous inputs,
but it will make automatic inferences in cases where the inputs are
unambiguous. This can apply when the times are supplied as a list of strings,
in which case it is not required to specify the format because the available
string formats have no overlap. However, if the format is known in advance
the string parsing will be faster if the format is provided.

The Time object maintains an internal representation of time as a pair of
double precision numbers expressing Julian days. The sum of the two numbers is
the Julian Date for that time relative to the given time scale. Users
requiring no better than microsecond precision over human time scales (~100
years) can safely ignore the internal representation details and skip this section.

This representation is driven by the underlying SOFA C-library implementation.
The SOFA routines take care throughout to maintain overall precision of the
double pair. The user is free to choose the way in which total JD is
distributed between the two values.

The internal JD pair is available via the jd1 and jd2 attributes.
Notice in the example below that when converting from UTC to TAI, the
small offset is placed in the jd2 value thus maintaining the highest
numeric precision:

The val argument is the only argument that is always required when creating a
Time object. This argument specifies the input time or times and
can be a single string or number, or it can be a Python list or numpy array
of strings or numbers.

In most situations one also needs to specify the time scale via the
scale argument. The Time class will never guess the time scale,
so a simple example would be:

The val2 argument is available for specialized situations where extremely
high precision is required. Recall that the internal representation of time
within astropy.time is two double-precision numbers that when summed give
the Julian date. If provided the val2 argument is used in combination with
val to set the second the internal time values. The exact interpretation of
val2 is determined by the input format class. As of this release all
string-valued formats ignore val2 and all numeric inputs effectively add
the two values in a way that maintains the highest precision. Example:

The scale argument sets the time scale and is required except for time
formats such as ‘cxcsec’ (TimeCxcSec) and ‘unix’
(TimeUnix). These formats represent the duration
in SI seconds since a fixed instant in time which is independent of time scale.

The precision setting affects string formats when outputting a value that
includes seconds. It must be an integer between 0 and 9. There is no effect
when inputting time values from strings. The default precision is 3. Note
that the limit of 9 digits is driven by the way that SOFA handles fractional
seconds. In practice this should should not be an issue.

The in_subfmt argument provides a mechanism to select one or more
subformat values from the available subformats for string input. Multiple
allowed subformats can be selected using Unix-style wildcard characters, in
particular * and ?, as documented in the Python fnmatch module.

The default value for in_subfmt is * which matches any available
subformat. This allows for convenient input of values with unknown or
heterogeneous subformat:

These optional parameters specify the observer latitude and longitude in
decimal degrees. They default to 0.0 and are used for time scales that are
sensitive to observer position. Currently the only time scale for which this
applies is TDB, which relies on the SOFA routine iauDtdb to determine the
time offset between TDB and TT.

Instants of time can be represented in different ways, for instance as an
ISO-format date string ('1999-07-2304:31:00') or seconds since 1998.0
(49091460.0) or Modified Julian Date (51382.187451574).

The representation of a Time object in a particular format is available
by getting the object attribute corresponding to the format name. The list of
available format names is in the time format section.

>>> t=Time('2010-01-01 00:00:00',format='iso',scale='utc')>>> t.jd# JD representation of time in current scale (UTC)2455197.5>>> t.iso# ISO representation of time in current scale (UTC)'2010-01-01 00:00:00.000'>>> t.unix# seconds since 1970.0 (UTC)1262304000.0>>> t.cxcsec# SI seconds since 1998.0 (TT)378691266.184

A new Time object for the same time value(s) but referenced to a new time
scale can be created getting the object attribute corresponding to the time
scale name. The list of available time scale names is in the time scale
section and in the figure below illustrating the network of time scale
transformations.

In this process the format and other object attributes like lat,
lon, and precision are also propagated to the new object.

As noted in the Timeobjectbasics section, a Time object is immutable and
the internal time values cannot be altered once the object is created. The
process of changing the time scale therefore begins by making a copy of the
original object and then converting the internal time values in the copy to the
new time scale. The new Time object is returned by the attribute access.

Time scale transformations that cross one of the orange circles in the image
above require an additional offset time value that is model or
observation-dependent. See SOFA Time Scale and Calendar Tools for further details.

The two attributes delta_ut1_utc and
delta_tdb_tt provide a way to set
these offset times explicitly. These represent the time scale offsets
UT1 - UTC and TDB - TT, respectively. As an example:

In the case of the TDB to TT offset, most users need only provide the lat
and lon values when creating the Time object. If the
delta_tdb_tt attribute is not explicitly set
then the SOFA C-library routine iauDtdb will be used to compute the
TDB to TT offset. Note that lat and lon are initialized to 0.0 by
default, so those defaults will be used if they are not provided.

The following code replicates an example in the SOFA Time Scale and Calendar
Tools document. It
does the transform from UTC to all supported time scales (TAI, TCB, TCG, TDB,
TT, UT1, UTC). This requires auxilliary information (latitude and longitude).

Simple time arithmetic is supported using via the TimeDelta class. The
following operations are available:

Create a TimeDelta explicitly by instantiating a class object

Create a TimeDelta by subtracting two Times

Add a TimeDelta to a Time object to get a new Time

Subtract a TimeDelta from a Time object to get a new Time

Add two TimeDelta objects to get a new TimeDelta

The TimeDelta class is derived from the Time class and shares many of its
properties. The key difference is that the time scale is always TAI so that
all time deltas are referenced to a uniform Julian Day which is exactly 86400
standard SI seconds.

This package makes use of the SOFA Software ANSI C library. The copyright of the SOFA
Software belongs to the Standards Of Fundamental Astronomy Board of the
International Astronomical Union. This library is made available under the
terms of the SOFA license.